ol truck has 245k . pressure was next to nothing . Sending unit was leaking a little so I replaced it . oil pressure went right up to where it should be . After about 15 miles when you stop at a sign the gauge jumps around at the zero mark . While stopped just slight pressure on the acelerator it is right where it should be . stop and put it in park and pressure is fine , but gauge jumps a little . The problem is when it is in gear stopped or low idle . I have not put a mechanical gauge on it .
Spring weak in the pump ? pick up tube ? Time to put a pump in ?

Thank you

 

With that many miles, I'd be more inclined to pull the engine apart and put new lower bearings in it. But there's problems with that too. The oil system primarily works by sending oil thru the crankshaft and that provides lube for the lower bearings. The piston rod caps on the bottom of the crank have slits in them to spray oil on the cylinder walls and bottom of the pistons as the crank rotates. As an engine wears, more oil is allowed to run out of the bearing and doesn't get sprayed on the cylinder walls in enough volume to prevent accelerated ring wear. It's a downhill kinda thing. So renewing the lower bearings will boost the oil pressure back to normal but now you have worn rings that won't wipe the oil away from the cylinder walls and the engine uses oil like it was a camel on water.
I'd suggest a mechanical gauge and if you have odd oil pressure, you're looking at a rebuild. No low pressure with a mechanical gauge and I'd be looking at a new oil pump. I'm a fan of a high volume, not high pressure pump if replacing it. If you want to cut a corner on this thing, change the oil to Delo 400 in 15w-40 and see if that buys you anything. The added viscosity can boost pressure at the bearings and Delo has the best formula for detergents to clean anything up like the oil pump screen. Use a Motorcraft filter. Hopefully that helps a lot.